The targetable or homing drill is a drill which can be directed to follow a certain path or can signal deviations from this path, or can automatically compensate for deviations from a predetermined path so that the orientation of the drill is automatically adjusted.
Such a drill generally comprises a bit or cutter which can be mounted upon an inner tube rotatable in an outer tube at the leading end of the drill string or head, the head being connected by other lengths of tube to a source of a drilling fluid which is piped through the drill string to the head.
The rotation of the inner tube by an external drive or the rotation of the bit or cutter by an electric motor, allows the bit to cut away rock strata into which the drill advances and the drilling detritus and spent mud can pass through channels between ribs of an outer tube of the head to the mouth of the bore. The outer tube can be provided with means for adjusting the orientation of the head to ensure drilling along a predetermined path.
Such drills can also be provided with telemetering facilities enabling parameters of drill operation to be transmitted to a remote location for evaluation.
The information which is thus telemetered to the evaluation station can comprise parameters detected by sensors in the drill and can represent parameters of the drilling operation, i.e. the drilling direction or deviations therefrom, as well as parameters related to the functions of the various devices or components of the drill string.
For example, when the drill string is of the direction-correcting type, direction correction can be effected by ribs which may be swingably mounted on the outer tube of the drill string and which can be actuated by hydraulic means, e.g. individual cylinders in the outer tube, under the control of a hydraulic circuit. It is important to monitor the operations of these hydraulic elements and the hydraulic circuit as well.
Reference may be had to a known correcting drill, namely that described in German patent document-open application DE-OS No. 30 00 239.2. In the outer tube of this drill string for control of the hydraulically actuatable cylinder for the control ribs, a plurality, preferably two, inclinometers are provided and are oriented in two mutually perpendicular vertical measuring planes.
The measurements from these inclinometers not only serve as the actual value signals for automatic control of the ribs and hence the orientation of the string during further drilling but are transmitted by telemetering to a station at the mouth of the bore. The telemetering is here effected by electrical signals which are transmitted through cable and trained along with the drill or are transmitted via conductors formed in the drilling tubes. These signals are highly precise but the system has the disadvantage that contact between the tubes may be problematical and there is a danger to a cable entrained along with the drill so that for mechanical reasons this earlier system has been found to be unreliable.
In German patent document-open application DE-OS No. 29 41 102, a rotary drill string is described in which the telemetering device utilizes a hydraulic converter which transforms electrical signals into pressure pulses of the flushing string, the pressure-modulated flushing string serving as a carrier for pressure signals. However, the recognition of these signals is poor in the prior art device, the ability to transmit data is limited and generally the system is ineffective because sharp rising and falling flanks are not observed on the pressure pulses.